1. Field of the Invention
The present invention relates to a power-supply unit which incorporates an electric double layer capacitor. The present invention also relates to a method of charging the electric double layer capacitor.
2. Description of the Related Art
The electric double layer capacitor is a type of large-capacity capacitor, and features of the electric double layer capacitor include: a boost charge is possible in comparison with a secondary battery; service life is long; and category temperature range is wide. The electric double layer capacitor is often used as a backup power supply for an electronic device in combination with a battery.
When the electric double layer capacitor is connected in parallel to the battery, a plurality of electric double layer capacitors must be used and connected in series so that the sum of the withstand voltages of all the electric double layer capacitors exceeds the terminal voltage across the battery since the withstand voltage of the electric double layer capacitor is generally low (1xcx9c3 volts). In addition, when the electric double layer capacitor is charged with a power supply (a battery voltage) which provides voltage higher than the withstand voltage of the electric double layer capacitor, the battery voltage must be decreased by, e.g., a potential transformer, to be lower than the withstand voltage of the electric double layer capacitor before being supplied to the electric double layer capacitor. This results in an increase in size of an electronic device which incorporates the electric double layer capacitor, and also an increase in the cost of production of the electronic device.
An object of the present invention is to provide a power-supply unit which incorporates an electric double layer capacitor used in combination with a battery providing voltage higher than the withstand voltage of the electric double layer capacitor, wherein the electric double layer capacitor can be easily charged with a simple system. Another object of the present invention is to provide a method of charging such an electric double layer capacitor.
To achieve the object mentioned above, according to an aspect of the present invention, a power-supply unit is provided, including a plurality of cells for supplying power to a load; a capacitor connected in parallel to the plurality of cells; at least one voltage detector which detects a terminal voltage across each of the plurality of cells; a selecting device for selecting a first combination from predetermined combinations of at least one of the plurality of cells in accordance with the terminal voltage, wherein the selected first combination of cells can supply a maximum voltage for charging the capacitor without exceeding a withstand voltage of the capacitor; and a controller for charging the electric double layer capacitor with the selected first combination of cells.
In an embodiment, the capacitor is an electric double layer capacitor.
In an embodiment the selecting device selects the first combination from the predetermined combinations of at least one of the plurality of cells which are connected in series in order with respect to ground, so that a total voltage across the first combination of cells connected in series becomes maximum without exceeding the withstand voltage.
In an embodiment, the plurality of cells are connected in series, wherein the power-supply unit further includes a switch system for connecting each cell included in the first combination of cells to the electric double layer capacitor in parallel. The controller controls the switch system so that each cell included in the first combination of cells is connected in parallel to the electric double layer capacitor.
In an embodiment, the selecting device determines a cell from the plurality of cells which has the highest terminal voltage among the plurality of cells, and selects a second combination from the cell which has the highest terminal voltage among said plurality of cells or predetermined combinations of the cell which has the highest terminal voltage among said plurality of cells and at least one of the remaining cells of the plurality of cells; wherein the total voltage across the second combination of cells connected in series becomes maximum without exceeding a withstand voltage of the electric double layer capacitor.
In an embodiment, the selecting device selects the second combination from the predetermined combinations, wherein the total voltage across the second combination of cells connected in series becomes maximum without exceeding the withstand voltage of the electric double layer capacitor.
In an embodiment, the number of cells included in the second combination of cells is minimal, and the cells included in the second combination are selected in decreasing order of terminal voltage
In an embodiment, the priority of selection of the second combination is firstly the closest combination to the withstand voltage, and secondly, the minimal number of cells.
In an embodiment, a second cell among the plurality of cells is removed from the predetermined combinations of cells if a voltage difference between the cell which has the highest terminal voltage among the plurality of cells and the second cell is greater than a reference value.
In an embodiment, the power-supply unit further includes a first switch system for connecting the plurality of cells in series in predetermined combinations, and a second switch system for connecting each of the predetermined combinations of cells to the electric double layer capacitor in parallel. The controller controls the first switch system so that cells included in the second combination are connected in series, and controls the second switch system so that the cells included in the second combination which are connected in series are connected in parallel with respect to the electric double layer capacitor.
In an embodiment, a step-up transformer is further provided which steps up a voltage output from the selected first combination of cells connected in series to a predetermined voltage level. The controller drives the step-up transformer to step up the voltage output from the selected combination of cells connected in series to supply the stepped up voltage to the load when the electric double layer capacitor is charged.
In an embodiment, the controller drives the step-up transformer to step up the voltage output from the selected first combination of cells connected in series to supply the stepped up voltage to the load when a total voltage of the plurality of cells is lower than a predetermined threshold value.
Preferably, an indicator is further provided for indicating various pieces of information; if a total voltage of the plurality of cells is lower than a predetermined threshold value, the controller sends signals to the indicator to display an indication for informing a user that there is not sufficient battery power to charge the electric double layer capacitor, and prohibits the electric double layer capacitor from being charged.
Preferably, the controller sends signals to the indicator to display an indication for warning a user that a particular cell included in the plurality of cells needs to be renewed, in the case where the terminal voltage across the particular cell which is detected via the at least one voltage detector is lower than a predetermined threshold value.
According to an aspect of the present invention, a power-supply unit is provided, including a battery chamber which accommodates a plurality of cells and includes a plurality of pairs of contact terminals, wherein each pair of contact terminals of the plurality of pairs of contact terminals respectively contact positive and negative electrodes of a corresponding one of the plurality of cells; an electric double layer capacitor which can be connected in parallel to the plurality of cells via the plurality of pairs of contact terminals; at least one voltage detector which detects a terminal voltage across each of the plurality of cells via the plurality of pairs of contact terminals; a selecting device for selecting a combination from predetermined possible combinations of serially-connected contact terminals included in the plurality of pairs of contact terminals in accordance with the terminal voltage, wherein a combination of cells of the plurality of cells which is determined by the selected possible combination of serially-connected contact terminals can supply a maximum voltage for charging the electric double layer capacitor without exceeding a withstand voltage of the electric double layer capacitor; and a controller which connects one and another terminals of the selected combination of serially-connected contact terminals to one and another terminals of the electric double layer capacitor.
According to another aspect of the present invention, a method is provided for charging an electric double layer capacitor provided in a power-supply unit in which at least two of a plurality of cells are connected in series to be connected in parallel to the electric double layer capacitor to supply power from the serially-connected at least two cells to a load. The charging method includes detecting a terminal voltage across each of the plurality of cells; selecting a combination from predetermined combinations of at least one of the plurality of cells in accordance with the terminal voltage, wherein the selected combination of cells can supply a maximum voltage for charging the electric double layer capacitor without exceeding a withstand voltage of the electric double layer capacitor; and connecting the selected combination of cells to the electric double layer capacitor in parallel to charge the electric double layer capacitor with the selected combination of cells.
Preferably, the at least one of the plurality of cells are connected in series in order with respect to ground, so that a total voltage across the combination of cells connected in series becomes maximum without exceeding the withstand voltage.
In an embodiment, the charging method further includes determining a cell from the plurality of cells which has the highest terminal voltage among the plurality of cells, and selecting a second combination from predetermined combinations of the determined cell and at least one of the remaining cells of the plurality of cells, wherein the total voltage across the second combination of cells connected in series becomes maximum without exceeding a withstand voltage of the electric double layer capacitor.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-103170 (filed on Apr. 5, 2000) which is expressly incorporated herein by reference in its entirety.